Method for triggering power headroom reporting in a dual connectivity system and a device therefor

ABSTRACT

A method is provided for a user equipment (UE) operating in a wireless communication system. The UE is configured with a first medium access control (MAC) entity and a second MAC entity. The UE triggers a power headroom reporting (PHR) in the first MAC entity if a condition is satisfied. A PHR MAC control element (CE) is generated for the triggered PHR. The UE transmits the PHR MAC CE to a first base station. The condition includes: there is an uplink (UL) transmission on any cell of the second MAC entity, and a required power backoff for the any cell of the second MAC entity has changed more than a threshold.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.14/657,749 filed on Mar. 13, 2015 (now U.S. Pat. No. 9,655,066 issued onMay 16, 2017), which claims the benefit of the U.S. Provisional PatentApplication 61/952,876 filed on Mar. 14, 2014, all of which are herebyexpressly incorporated by reference into the present application.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a wireless communication system and,more particularly, to a method for triggering power headroom reportingin a dual connectivity system and a device therefor.

Discussion of the Related Art

As an example of a mobile communication system to which the presentinvention is applicable, a 3rd Generation Partnership Project Long TermEvolution (hereinafter, referred to as LTE) communication system isdescribed in brief.

FIG. 1 is a view schematically illustrating a network structure of anE-UMTS as an exemplary radio communication system. An Evolved UniversalMobile Telecommunications System (E-UMTS) is an advanced version of aconventional Universal Mobile Telecommunications System (UMTS) and basicstandardization thereof is currently underway in the 3GPP. E-UMTS may begenerally referred to as a Long Term Evolution (LTE) system. For detailsof the technical specifications of the UMTS and E-UMTS, reference can bemade to Release 7 and Release 8 of “3rd Generation Partnership Project;Technical Specification Group Radio Access Network”.

Referring to FIG. 1, the E-UMTS includes a User Equipment (UE), eNode Bs(eNBs), and an Access Gateway (AG) which is located at an end of thenetwork (E-UTRAN) and connected to an external network. The eNBs maysimultaneously transmit multiple data streams for a broadcast service, amulticast service, and/or a unicast service.

One or more cells may exist per eNB. The cell is set to operate in oneof bandwidths such as 1.25, 2.5, 5, 10, 15, and 20 MHz and provides adownlink (DL) or uplink (UL) transmission service to a plurality of UEsin the bandwidth. Different cells may be set to provide differentbandwidths. The eNB controls data transmission or reception to and froma plurality of UEs. The eNB transmits DL scheduling information of DLdata to a corresponding UE so as to inform the UE of a time/frequencydomain in which the DL data is supposed to be transmitted, coding, adata size, and hybrid automatic repeat and request (HARQ)-relatedinformation. In addition, the eNB transmits UL scheduling information ofUL data to a corresponding UE so as to inform the UE of a time/frequencydomain which may be used by the UE, coding, a data size, andHARQ-related information. An interface for transmitting user traffic orcontrol traffic may be used between eNBs. A core network (CN) mayinclude the AG and a network node or the like for user registration ofUEs. The AG manages the mobility of a UE on a tracking area (TA) basis.One TA includes a plurality of cells.

Although wireless communication technology has been developed to LTEbased on wideband code division multiple access (WCDMA), the demands andexpectations of users and service providers are on the rise. Inaddition, considering other radio access technologies under development,new technological evolution is required to secure high competitivenessin the future. Decrease in cost per bit, increase in serviceavailability, flexible use of frequency bands, a simplified structure,an open interface, appropriate power consumption of UEs, and the likeare required.

SUMMARY OF THE INVENTION

The object of the present invention can be achieved by providing amethod for a User Equipment (UE) operating in a wireless communicationsystem, the method comprising: configuring a plurality of Medium AccessControl (MAC) entities, each MAC entity corresponding to eachenhanced-NodeB (eNB); occurring an event for triggering power headroomreporting (PHR) in a MAC entity of the plurality of MAC entities;triggering the PHR in the plurality of MAC entities including the MACentity if a type of the event is a first type event; and triggering thePHR in the MAC entity if the type of the event is a second type event.

In another aspect of the present invention, provided herein is a methodfor a User Equipment (UE) operating in a wireless communication system,the method comprising: configuring a first Medium Access Control (MAC)entity and a second MAC entity; and triggering power headroom reporting(PHR) in the second MAC entity if a prohibit-PHR-timer for the first MACentity expires

In another aspect of the present invention, provided herein is a methodfor a User Equipment (UE) operating in a wireless communication system,the method comprising: configuring a first Medium Access Control (MAC)entity and a second MAC entity; and triggering power headroom reporting(PHR) in the second MAC entity if at least one SCell for the first MACentity is changed from deactivate status to activate status.

Preferably, the first type event comprises at least one of the followingevents: a prohibitPHR-Timer of the MAC entity expires or has expired andpath loss has changed since the last PHR is larger than a threshold, anSCell belonging to the MAC entity with configured uplink is activated,or the prohibitPHR-Timer of the MAC entity expires or has expired whenthe UE has UL resources for new transmission to a corresponding eNB.

Preferably, the second type event comprises at least one of thefollowing events: a periodicPHR-Timer of the MAC entity expires, or uponconfiguration or reconfiguration of the power headroom reportingfunctionality for the MAC entity by upper layers, which is not used todisable the function.

Preferably, the first type event is related to cell status change.

Preferably, the first type event further comprises an event that aPSCell (Primary Secondary Cell) is added.

Preferably, the method further comprises: generating a plurality of PHRMAC CEs (Control Elements) if the type of the event is the first typeevent, wherein the each PHR MAC CE is for each MAC entity; andtransmitting each of the plurality of PHR MAC CEs to respective eNBs.

Preferably, the method further comprises: generating a PHR MAC CE(Control Elements) for the MAC entity if the type of the event is thesecond type event; and transmitting the PHR MAC CE to corresponding eNB.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention.

FIG. 1 is a diagram showing a network structure of an Evolved UniversalMobile Telecommunications System (E-UMTS) as an example of a wirelesscommunication system;

FIG. 2A is a block diagram illustrating network structure of an evolveduniversal mobile telecommunication system (E-UMTS), and FIG. 2B is ablock diagram depicting architecture of a typical E-UTRAN and a typicalEPC;

FIG. 3 is a diagram showing a control-plane protocol stack and auser-plane protocol stack of a radio interface protocol between a UE andan E-UTRAN based on a 3rd generation partnership project (3GPP) radioaccess network standard;

FIG. 4 is a diagram of an example physical channel structure used in anE-UMTS system;

FIG. 5 is a block diagram of a communication apparatus according to anembodiment of the present invention;

FIG. 6 is a diagram for carrier aggregation;

FIG. 7 is a conceptual diagram for dual connectivity between a MasterCell Group (MCG) and a Secondary Cell Group (SCG);

FIG. 8A is a conceptual diagram for C-Plane connectivity of basestations involved in dual connectivity, and FIG. 8B is a conceptualdiagram for U-Plane connectivity of base stations involved in dualconnectivity;

FIG. 9 is a conceptual diagram for radio protocol architecture for dualconnectivity;

FIG. 10 is a diagram for a general overview of the LTE protocolarchitecture for the downlink;

FIG. 11 is a diagram for signaling of buffer status and power-headroomreports;

FIG. 12 is a conceptual diagram for PHR MAC CE (Power Headroom ReportingMAC Control CE);

FIG. 13 is a conceptual diagram for Extended PHR MAC CE (Power HeadroomReporting MAC Control CE).

FIG. 14 is a conceptual diagram for triggering power headroom reportingin a dual connectivity system according to embodiments of the presentinvention; and

FIGS. 15 and 16 are conceptual diagrams for triggering power headroomreporting corresponding to a first type according to embodiments of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Universal mobile telecommunications system (UMTS) is a 3rd Generation(3G) asynchronous mobile communication system operating in wideband codedivision multiple access (WCDMA) based on European systems, globalsystem for mobile communications (GSM) and general packet radio services(GPRS). The long-term evolution (LTE) of UMTS is under discussion by the3rd generation partnership project (3GPP) that standardized UMTS.

The 3GPP LTE is a technology for enabling high-speed packetcommunications. Many schemes have been proposed for the LTE objectiveincluding those that aim to reduce user and provider costs, improveservice quality, and expand and improve coverage and system capacity.The 3G LTE requires reduced cost per bit, increased serviceavailability, flexible use of a frequency band, a simple structure, anopen interface, and adequate power consumption of a terminal as anupper-level requirement.

Hereinafter, structures, operations, and other features of the presentinvention will be readily understood from the embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings. Embodiments described later are examples in which technicalfeatures of the present invention are applied to a 3GPP system.

Although the embodiments of the present invention are described using along term evolution (LTE) system and a LTE-advanced (LTE-A) system inthe present specification, they are purely exemplary. Therefore, theembodiments of the present invention are applicable to any othercommunication system corresponding to the above definition. In addition,although the embodiments of the present invention are described based ona frequency division duplex (FDD) scheme in the present specification,the embodiments of the present invention may be easily modified andapplied to a half-duplex FDD (H-FDD) scheme or a time division duplex(TDD) scheme.

FIG. 2A is a block diagram illustrating network structure of an evolveduniversal mobile telecommunication system (E-UMTS). The E-UMTS may bealso referred to as an LTE system. The communication network is widelydeployed to provide a variety of communication services such as voice(VoIP) through IMS and packet data.

As illustrated in FIG. 2A, the E-UMTS network includes an evolved UMTSterrestrial radio access network (E-UTRAN), an Evolved Packet Core (EPC)and one or more user equipment. The E-UTRAN may include one or moreevolved NodeB (eNodeB) 20, and a plurality of user equipment (UE) 10 maybe located in one cell. One or more E-UTRAN mobility management entity(MME)/system architecture evolution (SAE) gateways 30 may be positionedat the end of the network and connected to an external network.

As used herein, “downlink” refers to communication from eNodeB 20 to UE10, and “uplink” refers to communication from the UE to an eNodeB. UE 10refers to communication equipment carried by a user and may be alsoreferred to as a mobile station (MS), a user terminal (UT), a subscriberstation (SS) or a wireless device.

FIG. 2B is a block diagram depicting architecture of a typical E-UTRANand a typical EPC.

As illustrated in FIG. 2B, an eNodeB 20 provides end points of a userplane and a control plane to the UE 10. MME/SAE gateway 30 provides anend point of a session and mobility management function for UE 10. TheeNodeB and MME/SAE gateway may be connected via an S1 interface.

The eNodeB 20 is generally a fixed station that communicates with a UE10, and may also be referred to as a base station (BS) or an accesspoint. One eNodeB 20 may be deployed per cell. An interface fortransmitting user traffic or control traffic may be used between eNodeBs20.

The MME provides various functions including NAS signaling to eNodeBs20, NAS signaling security, AS Security control, Inter CN node signalingfor mobility between 3GPP access networks, Idle mode UE Reachability(including control and execution of paging retransmission), TrackingArea list management (for UE in idle and active mode), PDN GW andServing GW selection, MME selection for handovers with MME change, SGSNselection for handovers to 2G or 3G 3GPP access networks, Roaming,Authentication, Bearer management functions including dedicated bearerestablishment, Support for PWS (which includes ETWS and CMAS) messagetransmission. The SAE gateway host provides assorted functions includingPer-user based packet filtering (by e.g. deep packet inspection), LawfulInterception, UE IP address allocation, Transport level packet markingin the downlink, UL and DL service level charging, gating and rateenforcement, DL rate enforcement based on APN-AMBR. For clarity MME/SAEgateway 30 will be referred to herein simply as a “gateway,” but it isunderstood that this entity includes both an MME and an SAE gateway.

A plurality of nodes may be connected between eNodeB 20 and gateway 30via the S1 interface. The eNodeBs 20 may be connected to each other viaan X2 interface and neighboring eNodeBs may have a meshed networkstructure that has the X2 interface.

As illustrated, eNodeB 20 may perform functions of selection for gateway30, routing toward the gateway during a Radio Resource Control (RRC)activation, scheduling and transmitting of paging messages, schedulingand transmitting of Broadcast Channel (BCCH) information, dynamicallocation of resources to UEs 10 in both uplink and downlink,configuration and provisioning of eNodeB measurements, radio bearercontrol, radio admission control (RAC), and connection mobility controlin LTE_ACTIVE state. In the EPC, and as noted above, gateway 30 mayperform functions of paging origination, LTE-IDLE state management,ciphering of the user plane, System Architecture Evolution (SAE) bearercontrol, and ciphering and integrity protection of Non-Access Stratum(NAS) signaling.

The EPC includes a mobility management entity (MME), a serving-gateway(S-GW), and a packet data network-gateway (PDN-GW). The MME hasinformation about connections and capabilities of UEs, mainly for use inmanaging the mobility of the UEs. The S-GW is a gateway having theE-UTRAN as an end point, and the PDN-GW is a gateway having a packetdata network (PDN) as an end point.

FIG. 3 is a diagram showing a control-plane protocol stack and a userplane protocol stack of a radio interface protocol between a UE and anE-UTRAN based on a 3GPP radio access network standard. As shown in FIG.3(a), the control-plane protocol stack refers to a path used fortransmitting control messages used for managing a call between the UEand the E-UTRAN. As shown in FIG. 3(b), the user-plane protocol stackrefers to a path used for transmitting data generated in an applicationlayer, e.g., voice data or Internet packet data.

A physical (PHY) layer of a first layer provides an information transferservice to a higher layer using a physical channel. The PHY layer isconnected to a medium access control (MAC) layer located on the higherlayer via a transport channel. Data is transported between the MAC layerand the PHY layer via the transport channel. Data is transported betweena physical layer of a transmitting side and a physical layer of areceiving side via physical channels. The physical channels use time andfrequency as radio resources. In detail, the physical channel ismodulated using an orthogonal frequency division multiple access (OFDMA)scheme in downlink and is modulated using a single carrier frequencydivision multiple access (SC-FDMA) scheme in uplink.

The MAC layer of a second layer provides a service to a radio linkcontrol (RLC) layer of a higher layer via a logical channel. The RLClayer of the second layer supports reliable data transmission. Afunction of the RLC layer may be implemented by a functional block ofthe MAC layer. A packet data convergence protocol (PDCP) layer of thesecond layer performs a header compression function to reduceunnecessary control information for efficient transmission of anInternet protocol (IP) packet such as an IP version 4 (IPv4) packet oran IP version 6 (IPv6) packet in a radio interface having a relativelysmall bandwidth.

A radio resource control (RRC) layer located at the bottom of a thirdlayer is defined only in the control plane. The RRC layer controlslogical channels, transport channels, and physical channels in relationto configuration, re-configuration, and release of radio bearers (RBs).An RB refers to a service that the second layer provides for datatransmission between the UE and the E-UTRAN. To this end, the RRC layerof the UE and the RRC layer of the E-UTRAN exchange RRC messages witheach other.

One cell of the eNB is set to operate in one of bandwidths such as 1.25,2.5, 5, 10, 15, and 20 MHz and provides a downlink or uplinktransmission service to a plurality of UEs in the bandwidth. Differentcells may be set to provide different bandwidths.

Downlink transport channels for transmission of data from the E-UTRAN tothe UE include a broadcast channel (BCH) for transmission of systeminformation, a paging channel (PCH) for transmission of paging messages,and a downlink shared channel (SCH) for transmission of user traffic orcontrol messages. Traffic or control messages of a downlink multicast orbroadcast service may be transmitted through the downlink SCH and mayalso be transmitted through a separate downlink multicast channel (MCH).

Uplink transport channels for transmission of data from the UE to theE-UTRAN include a random access channel (RACH) for transmission ofinitial control messages and an uplink SCH for transmission of usertraffic or control messages. Logical channels that are defined above thetransport channels and mapped to the transport channels include abroadcast control channel (BCCH), a paging control channel (PCCH), acommon control channel (CCCH), a multicast control channel (MCCH), and amulticast traffic channel (MTCH).

FIG. 4 is a view showing an example of a physical channel structure usedin an E-UMTS system. A physical channel includes several subframes on atime axis and several subcarriers on a frequency axis. Here, onesubframe includes a plurality of symbols on the time axis. One subframeincludes a plurality of resource blocks and one resource block includesa plurality of symbols and a plurality of subcarriers. In addition, eachsubframe may use certain subcarriers of certain symbols (e.g., a firstsymbol) of a subframe for a physical downlink control channel (PDCCH),that is, an L1/L2 control channel. In FIG. 4, an L1/L2 controlinformation transmission area (PDCCH) and a data area (PDSCH) are shown.In one embodiment, a radio frame of 10 ms is used and one radio frameincludes 10 subframes. In addition, one subframe includes twoconsecutive slots. The length of one slot may be 0.5 ms. In addition,one subframe includes a plurality of OFDM symbols and a portion (e.g., afirst symbol) of the plurality of OFDM symbols may be used fortransmitting the L1/L2 control information. A transmission time interval(TTI) which is a unit time for transmitting data is 1 ms.

A base station and a UE mostly transmit/receive data via a PDSCH, whichis a physical channel, using a DL-SCH which is a transmission channel,except a certain control signal or certain service data. Informationindicating to which UE (one or a plurality of UEs) PDSCH data istransmitted and how the UE receive and decode PDSCH data is transmittedin a state of being included in the PDCCH.

For example, in one embodiment, a certain PDCCH is CRC-masked with aradio network temporary identity (RNTI) “A” and information about datais transmitted using a radio resource “B” (e.g., a frequency location)and transmission format information “C” (e.g., a transmission blocksize, modulation, coding information or the like) via a certainsubframe. Then, one or more UEs located in a cell monitor the PDCCHusing its RNTI information. And, a specific UE with RNTI “A” reads thePDCCH and then receive the PDSCH indicated by B and C in the PDCCHinformation.

FIG. 5 is a block diagram of a communication apparatus according to anembodiment of the present invention.

The apparatus shown in FIG. 5 can be a user equipment (UE) and/or eNBadapted to perform the above mechanism, but it can be any apparatus forperforming the same operation.

As shown in FIG. 5, the apparatus may comprises a DSP/microprocessor(110) and RF module (transmiceiver; 135). The DSP/microprocessor (110)is electrically connected with the transceiver (135) and controls it.The apparatus may further include power management module (105), battery(155), display (115), keypad (120), SIM card (125), memory device (130),speaker (145) and input device (150), based on its implementation anddesigner's choice.

Specifically, FIG. 5 may represent a UE comprising a receiver (135)configured to receive a request message from a network, and atransmitter (135) configured to transmit the transmission or receptiontiming information to the network. These receiver and the transmittercan constitute the transceiver (135). The UE further comprises aprocessor (110) connected to the transceiver (135: receiver andtransmitter).

Also, FIG. 5 may represent a network apparatus comprising a transmitter(135) configured to transmit a request message to a UE and a receiver(135) configured to receive the transmission or reception timinginformation from the UE. These transmitter and receiver may constitutethe transceiver (135). The network further comprises a processor (110)connected to the transmitter and the receiver. This processor (110) maybe configured to calculate latency based on the transmission orreception timing information.

FIG. 6 is a diagram for carrier aggregation.

Carrier aggregation technology for supporting multiple carriers isdescribed with reference to FIG. 6 as follows. As mentioned in theforegoing description, it may be able to support system bandwidth up tomaximum 100 MHz in a manner of bundling maximum 5 carriers (componentcarriers: CCs) of bandwidth unit (e.g., 20 MHz) defined in a legacywireless communication system (e.g., LTE system) by carrier aggregation.Component carriers used for carrier aggregation may be equal to ordifferent from each other in bandwidth size. And, each of the componentcarriers may have a different frequency band (or center frequency). Thecomponent carriers may exist on contiguous frequency bands. Yet,component carriers existing on non-contiguous frequency bands may beused for carrier aggregation as well. In the carrier aggregationtechnology, bandwidth sizes of uplink and downlink may be allocatedsymmetrically or asymmetrically.

Multiple carriers (component carriers) used for carrier aggregation maybe categorized into primary component carrier (PCC) and secondarycomponent carrier (SCC). The PCC may be called P-cell (primary cell) andthe SCC may be called S-cell (secondary cell). The primary componentcarrier is the carrier used by a base station to exchange traffic andcontrol signaling with a user equipment. In this case, the controlsignaling may include addition of component carrier, setting for primarycomponent carrier, uplink (UL) grant, downlink (DL) assignment and thelike. Although a base station may be able to use a plurality ofcomponent carriers, a user equipment belonging to the corresponding basestation may be set to have one primary component carrier only. If a userequipment operates in a single carrier mode, the primary componentcarrier is used. Hence, in order to be independently used, the primarycomponent carrier should be set to meet all requirements for the dataand control signaling exchange between a base station and a userequipment.

Meanwhile, the secondary component carrier may include an additionalcomponent carrier that can be activated or deactivated in accordancewith a required size of transceived data. The secondary componentcarrier may be set to be used only in accordance with a specific commandand rule received from a base station. In order to support an additionalbandwidth, the secondary component carrier may be set to be usedtogether with the primary component carrier. Through an activatedcomponent carrier, such a control signal as a UL grant, a DL assignmentand the like can be received by a user equipment from a base station.Through an activated component carrier, such a control signal in UL as achannel quality indicator (CQI), a precoding matrix index (PMI), a rankindicator (RI), a sounding reference signal (SRS) and the like can betransmitted to a base station from a user equipment.

Resource allocation to a user equipment can have a range of a primarycomponent carrier and a plurality of secondary component carriers. In amulti-carrier aggregation mode, based on a system load (i.e.,static/dynamic load balancing), a peak data rate or a service qualityrequirement, a system may be able to allocate secondary componentcarriers to DL and/or UL asymmetrically. In using the carrieraggregation technology, the setting of the component carriers may beprovided to a user equipment by a base station after RRC connectionprocedure. In this case, the RRC connection may mean that a radioresource is allocated to a user equipment based on RRC signalingexchanged between an RRC layer of the user equipment and a network viaSRB. After completion of the RRC connection procedure between the userequipment and the base station, the user equipment may be provided bythe base station with the setting information on the primary componentcarrier and the secondary component carrier. The setting information onthe secondary component carrier may include addition/deletion (oractivation/deactivation) of the secondary component carrier. Therefore,in order to activate a secondary component carrier between a basestation and a user equipment or deactivate a previous secondarycomponent carrier, it may be necessary to perform an exchange of RRCsignaling and MAC control element.

The activation or deactivation of the secondary component carrier may bedetermined by a base station based on a quality of service (QoS), a loadcondition of carrier and other factors. And, the base station may beable to instruct a user equipment of secondary component carrier settingusing a control message including such information as an indication type(activation/deactivation) for DL/UL, a secondary component carrier listand the like.

FIG. 7 is a conceptual diagram for dual connectivity (DC) between aMaster Cell Group (MCG) and a Secondary Cell Group (SCG).

The dual connectivity means that the UE can be connected to both aMaster eNode-B (MeNB) and a Secondary eNode-B (SeNB) at the same time.The MCG is a group of serving cells associated with the MeNB, comprisingof a PCell and optionally one or more SCells. And the SCG is a group ofserving cells associated with the SeNB, comprising of the special SCelland optionally one or more SCells. The MeNB is an eNB which terminatesat least S1-MME (S1 for the control plane) and the SeNB is an eNB thatis providing additional radio resources for the UE but is not the MeNB.

The dual connectivity is a kind of carrier aggregation in that the UE isconfigured a plurality serving cells. However, unlike all serving cellssupporting carrier aggregation of FIG. 8 are served by a same eNB, allserving cells supporting dual connectivity of FIG. 10 are served bydifferent eNBs, respectively at same time. The different eNBs areconnected via non-ideal backhaul interface because the UE is connectedwith the different eNBs at same time.

With dual connectivity, some of the data radio bearers (DRBs) can beoffloaded to the SCG to provide high throughput while keeping schedulingradio bearers (SRBs) or other DRBs in the MCG to reduce the handoverpossibility. The MCG is operated by the MeNB via the frequency of f1,and the SCG is operated by the SeNB via the frequency of f2. Thefrequency f1 and f2 may be equal. The backhaul interface (BH) betweenthe MeNB and the SeNB is non-ideal (e.g. X2 interface), which means thatthere is considerable delay in the backhaul and therefore thecentralized scheduling in one node is not possible.

FIG. 8A shows C-plane (Control Plane) connectivity of eNBs involved indual connectivity for a certain UE: The MeNB is C-plane connected to theMME via S1-MME, the MeNB and the SeNB are interconnected viaX2-C(X2-Control plane). As FIG. 8a , Inter-eNB control plane signalingfor dual connectivity is performed by means of X2 interface signaling.Control plane signaling towards the MME is performed by means of S1interface signaling. There is only one S1-MME connection per UE betweenthe MeNB and the MME. Each eNB should be able to handle UEsindependently, i.e. provide the PCell to some UEs while providingSCell(s) for SCG to others. Each eNB involved in dual connectivity for acertain UE owns its radio resources and is primarily responsible forallocating radio resources of its cells, respective coordination betweenMeNB and SeNB is performed by means of X2 interface signaling.

FIG. 8B shows U-plane connectivity of eNBs involved in dual connectivityfor a certain UE. U-plane connectivity depends on the bearer optionconfigured: i) For MCG bearers, the MeNB is U-plane connected to theS-GW via S1-U, the SeNB is not involved in the transport of user planedata, ii) For split bearers, the MeNB is U-plane connected to the S-GWvia S1-U and in addition, the MeNB and the SeNB are interconnected viaX2-U, and iii) For SCG bearers, the SeNB is directly connected with theS-GW via S1-U. If only MCG and split bearers are configured, there is noS1-U termination in the SeNB. In the dual connectivity, enhancement ofthe small cell is required in order to data offloading from the group ofmacro cells to the group of small cells. Since the small cells can bedeployed apart from the macro cells, multiple schedulers can beseparately located in different nodes and operate independently from theUE point of view. This means that different scheduling node wouldencounter different radio resource environment, and hence, eachscheduling node may have different scheduling results.

FIG. 9 is a conceptual diagram for radio protocol architecture for dualconnectivity.

E-UTRAN of the present example can support dual connectivity operationwhereby a multiple receptions/transmissions (RX/TX) UE in RRC_CONNECTEDis configured to utilize radio resources provided by two distinctschedulers, located in two eNBs (or base stations) connected via anon-ideal backhaul over the X2 interface. The eNBs involved in dualconnectivity for a certain UE may assume two different roles: an eNB mayeither act as the MeNB or as the SeNB. In dual connectivity, a UE can beconnected to one MeNB and one SeNB.

In the dual connectivity operation, the radio protocol architecture thata particular bearer uses depends on how the bearer is setup. Threealternatives exist, MCG bearer (901), split bearer (903) and SCG bearer(905). Those three alternatives are depicted on FIG. 9. The SRBs(Signaling Radio Bearers) are always of the MCG bearer and thereforeonly use the radio resources provided by the MeNB. The MCG bearer (901)is a radio protocol only located in the MeNB to use MeNB resources onlyin the dual connectivity. And the SCG bearer (905) is a radio protocolonly located in the SeNB to use SeNB resources in the dual connectivity.

Specially, the split bearer (903) is a radio protocol located in boththe MeNB and the SeNB to use both MeNB and SeNB resources in the dualconnectivity and the split bearer (903) may be a radio bearer comprisingone Packet Data Convergence Protocol (PDCP) entity, two Radio LinkControl (RLC) entities and two Medium Access Control (MAC) entities forone direction. Specially, the dual connectivity operation can also bedescribed as having at least one bearer configured to use radioresources provided by the SeNB.

The expected benefits of the split bearer (903) are: i) the SeNBmobility hidden to CN, ii) no security impacts with ciphering beingrequired in MeNB only, iii) no data forwarding between SeNBs required atSeNB change, iv) offloads RLC processing of SeNB traffic from MeNB toSeNB, v) little or no impacts to RLC, vi) utilization of radio resourcesacross MeNB and SeNB for the same bearer possible, vii) relaxedrequirements for SeNB mobility (MeNB can be used in the meantime).

The expected drawbacks of the split bearer (903) are: i) need to route,process and buffer all dual connectivity traffic in the MeNB, ii) a PDCPentity to become responsible for routing PDCP PDUs towards eNBs fortransmission and reordering them for reception, iii) flow controlrequired between the MeNB and the SeNB, iv) in the uplink, logicalchannel prioritization impacts for handling RLC retransmissions and RLCStatus PDUs (restricted to the eNB where the corresponding RLC entityresides) and v) no support of local break-out and content caching atSeNB for dual connectivity UEs.

In dual connectivity, two MAC entities are configured in the UE: one forthe MCG and one for the SCG. Each MAC entity is configured by RRC with aserving cell supporting PUCCH transmission and contention based RandomAccess. The term SpCell refers to such cell, whereas the term SCellrefers to other serving cells. The term SpCell either refers to thePCell of the MCG or the PSCell (Primary Secondary Cell) of the SCGdepending on if the MAC entity is associated to the MCG or the SCG,respectively. A Timing Advance Group containing the SpCell of a MACentity is referred to as pTAG, whereas the term sTAG refers to otherTAGs.

The functions of the different MAC entities in the UE operateindependently if not otherwise indicated. The timers and parameters usedin each MAC entity are configured independently if not otherwiseindicated. The serving cells, C-RNTI, radio bearers, logical channels,upper and lower layer entities, LCGs, and HARQ entities considered byeach MAC entity refer to those mapped to that MAC entity if nototherwise indicated.

FIG. 10 is a diagram for a general overview of the LTE protocolarchitecture for the downlink.

A general overview of the LTE protocol architecture for the downlink isillustrated in FIG. 10. Furthermore, the LTE protocol structure relatedto uplink transmissions is similar to the downlink structure in FIG. 10,although there are differences with respect to transport formatselection and multi-antenna transmission.

Data to be transmitted in the downlink enters in the form of IP packetson one of the SAE bearers (1001). Prior to transmission over the radiointerface, incoming IP packets are passed through multiple protocolentities, summarized below and described in more detail in the followingsections:

-   -   Packet Data Convergence Protocol (PDCP, 1003) performs IP header        compression to reduce the number of bits necessary to transmit        over the radio interface. The header-compression mechanism is        based on ROHC, a standardized header-compression algorithm used        in WCDMA as well as several other mobile-communication        standards. PDCP (1003) is also responsible for ciphering and        integrity protection of the transmitted data. At the receiver        side, the PDCP protocol performs the corresponding deciphering        and decompression operations. There is one PDCP entity per radio        bearer configured for a mobile terminal.    -   Radio Link Control (RLC, 1005) is responsible for        segmentation/concatenation, retransmission handling, and        in-sequence delivery to higher layers. Unlike WCDMA, the RLC        protocol is located in the eNodeB since there is only a single        type of node in the LTE radio-access-network architecture. The        RLC (1005) offers services to the PDCP (1003) in the form of        radio bearers. There is one RLC entity per radio bearer        configured for a terminal.

There is one RLC entity per logical channel configured for a terminal,where each RLC entity is responsible for: i) segmentation,concatenation, and reassembly of RLC SDUs; ii) RLC retransmission; andiii) in-sequence delivery and duplicate detection for the correspondinglogical channel.

Other noteworthy features of the RLC are: (1) the handling of varyingPDU sizes; and (2) the possibility for close interaction between thehybrid-ARQ and RLC protocols. Finally, the fact that there is one RLCentity per logical channel and one hybrid-ARQ entity per componentcarrier implies that one RLC entity may interact with multiplehybrid-ARQ entities in the case of carrier aggregation.

The purpose of the segmentation and concatenation mechanism is togenerate RLC PDUs of appropriate size from the incoming RLC SDUs. Onepossibility would be to define a fixed PDU size, a size that wouldresult in a compromise. If the size were too large, it would not bepossible to support the lowest data rates. Also, excessive padding wouldbe required in some scenarios. A single small PDU size, however, wouldresult in a high overhead from the header included with each PDU. Toavoid these drawbacks, which is especially important given the verylarge dynamic range of data rates supported by LTE, the RLC PDU sizevaries dynamically.

In process of segmentation and concatenation of RLC SDUs into RLC PDUs,a header includes, among other fields, a sequence number, which is usedby the reordering and retransmission mechanisms. The reassembly functionat the receiver side performs the reverse operation to reassemble theSDUs from the received PDUs.

-   -   Medium Access Control (MAC, 1007) handles hybrid-ARQ        retransmissions and uplink and downlink scheduling. The        scheduling functionality is located in the eNodeB, which has one        MAC entity per cell, for both uplink and downlink. The        hybrid-ARQ protocol part is present in both the transmitting and        receiving end of the MAC protocol. The MAC (1007) offers        services to the RLC (1005) in the form of logical channels        (1009).    -   Physical Layer (PHY, 1011), handles coding/decoding,        modulation/demodulation, multi-antenna mapping, and other        typical physical layer functions. The physical layer (1011)        offers services to the MAC layer (1007) in the form of transport        channels (1013).

FIG. 11 is a diagram for signaling of buffer status and power-headroomreports.

The scheduler needs knowledge about an amount of data awaitingtransmission from the terminals to assign the proper amount of uplinkresources. Obviously, there is no need to provide uplink resources to aterminal with no data to transmit as this would only result in theterminal performing padding to fill up the granted resources. Hence, asa minimum, the scheduler needs to know whether the terminal has data totransmit and should be given a grant. This is known as a schedulingrequest.

The use of a single bit for the scheduling request is motivated by thedesire to keep the uplink overhead small, as a multi-bit schedulingrequest would come at a higher cost. A consequence of the single bitscheduling request is the limited knowledge at the eNodeB about thebuffer situation at the terminal when receiving such a request.Different scheduler implementations handle this differently. Onepossibility is to assign a small amount of resources to ensure that theterminal can exploit them efficiently without becoming power limited.Once the terminal has started to transmit on the UL-SCH, more detailedinformation about the buffer status and power headroom can be providedthrough the inband MAC control message, as discussed below.

Terminals that already have a valid grant obviously do not need torequest uplink resources. However, to allow the scheduler to determinethe amount of resources to grant to each terminal in future subframes,information about the buffer situation and the power availability isuseful, as discussed above. This information is provided to thescheduler as part of the uplink transmission through MAC controlelement. The LCID field in one of the MAC subheaders is set to areserved value indicating the presence of a buffer status report, asillustrated in FIG. 11.

Especially, to assist the scheduler in the selection of a combination ofmodulation-and-coding scheme and resource size M that does not lead tothe terminal being power limited, the terminal can be configured toprovide regular power headroom reports on its power usage. There is aseparate transmit-power limitation for each component carrier. Thus,power headroom should be measured and reported separately for eachcomponent carrier.

There are two different Types of power-headroom reports defined for LTErelease 10, Type 1 and Type 2. Type 1 reporting reflects the powerheadroom assuming PUSCH-only transmission on the carrier, while theType2 report assumes combined PUSCH and PUCCH transmission.

The Type1 power headroom valid for a certain subframe, assuming that theterminal was really scheduled for PUSCH transmission in that subframe,is given by the following expression:Power Headroom=P _(CMAX,c)−(P _(0,PUSCH) +α·PL_(DL)+10·log₁₀(M)+Δ_(MCS)+δ).  [Equation 1]

Where the values for M and ΔMCS correspond to the resource assignmentand modulation-and-coding scheme used in the subframe to which thepower-headroom report corresponds. It can be noted that the powerheadroom is not a measure of the difference between the maximumper-carrier transmit power and the actual carrier transmit power. It canbe seen that the power headroom is a measure of the difference betweenPCMAX,c and the transmit power that would have been used assuming thatthere would have been no upper limit on the transmit power. Thus, thepower headroom can very well be negative. More exactly, a negative powerheadroom indicates that the per-carrier transmit power was limited byPCMAX,c at the time of the power headroom reporting. As the networkknows what modulation-and-coding scheme and resource size the terminalused for transmission in the subframe to which the power-headroom reportcorresponds, it can determine what are the valid combinations ofmodulation-and-coding scheme and resource size M, assuming that thedownlink path loss PLDL and the term δ have not changed substantially.

Type 1 power headroom can also be reported for subframes where there isno actual PUSCH transmission. In such cases, 10*log 10 (M) and ΔMCS inthe expression above are set to zero:Power Headroom=P _(CMAX,c)−(P _(0,PUSCH) +α·PL _(DL)+δ).  [Equation 2]

This can be seen as the power headroom assuming a default transmissionconfiguration corresponding to the minimum possible resource assignment(M=1) and the modulation-and-coding scheme associated with Δ MCS=0 dB.

Similarly, Type2 power headroom reporting is defined as the differencebetween the maximum per-carrier transmit power and the sum of the PUSCHand PUCCH transmit power respectively, once again not taking intoaccount any maximum per-carrier power when calculating the PUSCH andPUCCH transmit power.

Similar to Type1 power headroom reporting, the Type2 power headroom canalso be reported for subframes in which no PUSCH and/or PUCCH istransmitted. In that case a virtual PUSCH and or PUCCH transmit power iscalculated, assuming the smallest possible resource assignment (M=1) andΔMCS=0 dB for PUSCH and Δ Format=0 for PUCCH.

For the uplink, the power availability, or power headroom is defined asthe difference between the nominal maximum output power and theestimated output power for UL-SCH transmission. This quantity can bepositive as well as negative (on a dB scale), where a negative valuewould indicate that the network has scheduled a higher data rate thanthe terminal can support given its current power availability. The powerheadroom depends on the power-control mechanism and thereby indirectlyon factors such as the interference in the system and the distance tothe base stations.

Information about the power headroom is fed back from the terminals tothe eNodeB in a similar way as the buffer-status reports—that is, onlywhen the terminal is scheduled to transmit on the UL-SCH. Type1 reportsare provided for all component carriers simultaneously, while Type2reports are provided for the primary component carrier only.

The PHR trigger events are defined for a MAC entity as follows:

-   -   Event 1: prohibitPHR-Timer expires or has expired and the path        loss has changed more than dl-PathlossChange dB for at least one        activated Serving Cell which is used as a pathloss reference        since the last transmission of a PHR when the UE has UL        resources for new transmission;    -   Event 2: periodicPHR-Timer expires;    -   Event 3: upon configuration or reconfiguration of the power        headroom reporting functionality by upper layers, which is not        used to disable the function;    -   Event 4: activation of an SCell with configured uplink.    -   Event 5: prohibitPHR-Timer expires or has expired, when the UE        has UL resources for new transmission, and the following is true        in this TTI for any of the actived Serving Cells with configured        uplink: there are UL resources allocated for transmission or        there is a PUCCH transmission on this cell, and the required        power backoff due to power management (as allowed by P-MPR_(c))        for this cell has changed more than dl-PathlossChange dB since        the last transmission of a PHR when the UE had UL resources        allocated for transmission or PUCCH transmission on this cell.

FIG. 12 is a conceptual diagram for PHR MAC CE (Power Headroom ReportingMAC Control CE).

The Power Headroom MAC control element is identified by a MAC PDUsubheader with LCID as specified in Table 1. It has a fixed size andconsists of a single octet defined as follows by Table 2.

TABLE 1 Index LCID values 00000 CCCH 00001-01010 Identity of the logicalchannel 01011-11000 Reserved 11001 Extended Power Headroom Report 11010Power Headroom Report 11011 C-RNTI 11100 Truncated BSR 11101 Short BSR11110 Long BSR 11111 Padding

TABLE 2 PH Power Headroom Level 0 POWER_HEADROOM_0 1 POWER_HEADROOM_1 2POWER_HEADROOM_2 3 POWER_HEADROOM_3 . . . . . . 60 POWER_HEADROOM_60 61POWER_HEADROOM_61 62 POWER_HEADROOM_62 63 POWER_HEADROOM_63

A field of R (1201) is a reserved bit and is set to “0”. And a field ofPH (1203) indicates the power headroom level. A length of the field ofPH (1203) is 6 bits. The reported PH and the corresponding powerheadroom levels are shown in Table 2 above.

FIG. 13 is a conceptual diagram for Extended PHR MAC CE (Power HeadroomReporting MAC Control CE).

The Extended Power Headroom MAC control element is identified by a MACPDU subheader with LCID as specified in the Table 1 above. It has avariable size and is defined in FIG. 12. When Type 2 PH is reported, theoctet containing the Type 2 PH field is included first after the octetindicating the presence of PH per SCell and followed by an octetcontaining the associated P_(CMAX,c) field (if reported). Then followsin ascending order based on the Serving CellIndex an octet with the Type1 PH field and an octet with the associated PCMAX,c field (if reported),for the PCell and for each SCell indicated in the bitmap.

The Extended Power Headroom MAC Control Element is defined as follows:

A field of ‘Ci’ (1301) indicates the presence of a PH field for theSCell with SCell index i. The Ci field set to “1” indicates that a PHfield for the SCell with SCell index i is reported. The Ci field set to“0” indicates that a PH field for the SCell with SCell index i is notreported;

A field of ‘R’ (1303) is a reserved bit and is set to “0”;

A field of ‘V’ (1305) indicates if the PH value is based on a realtransmission or a reference format. For Type 1 PH, V=0 indicates realtransmission on PUSCH and V=1 indicates that a PUSCH reference format isused. For Type 2 PH, V=0 indicates real transmission on PUCCH and V=1indicates that a PUCCH reference format is used. Furthermore, for bothType 1 and Type 2 PH, V=0 indicates the presence of the octet containingthe associated P_(CMAX,c) field, and V=1 indicates that the octetcontaining the associated P_(CMAX,c) field is omitted;

A field of ‘PH’ (1307) indicates the power headroom level. The length ofthe field is 6 bits. The reported PH and the corresponding powerheadroom levels are shown in Table 2

A field of ‘P’ (1309) indicates whether the UE applies power backoff dueto power management as allowed by P-MPR_(c). The UE shall set P=1 if thecorresponding P_(CMAX,c) field would have had a different value if nopower backoff due to power management had been applied;

A field of ‘P_(CMAX,c)’(1311) if present, this field indicates theP_(CMAX,c) or {tilde over (P)}_(CMAX,c) used for calculation of thepreceding PH field. The reported P_(CMAX,c) and the correspondingnominal UE transmit power levels are shown in Table 3.

If the UE has UL resources allocated for new transmission for a TTI andif the UE is configured to simultaneous PUCCH-PUSCH transmission, theExtended Power Headroom MAC Control Element includes a value of a Type 2power headroom for the cell in the field of PH in a TTI. In this case,if the UE has a PUCCH transmission in the TTI, the Extended PowerHeadroom MAC Control Element includes a value of the correspondingP_(CMAX,c) field from a physical layer;

TABLE 3 PH Power Headroom Level 0 POWER_HEADROOM_0 1 POWER_HEADROOM_1 2POWER_HEADROOM_2 3 POWER_HEADROOM_3 . . . . . . 60 POWER_HEADROOM_60 61POWER_HEADROOM_61 62 POWER_HEADROOM_62 63 POWER_HEADROOM_63

The UE reports the power headroom (PH) information to the eNB in orderfor the eNB to allocate more resource to the UE unless the allocatedresource do not exceed the amount of resource that UE can support giventhe UE's maximum transmission power. In detail, the UE reports allactivated serving cells configured for the UE to the eNB using MACsignaling, i.e., PHR MAC CE. When the UE reports the PH information ofcells to the corresponding eNB, as the eNB is aware of the schedulinginformation as well as the power headroom of the UE, the eNB can assessthe exact power status of the UE.

In dual connectivity, as the UE is connected to more than one eNBs andscheduled by them, the UE has separate MAC entities correspond to eacheNB. Thus, PHR operation is re-considered in the following aspects: PHRconfiguration, PHR triggering, PHR transmission.

1) PHR configuration: as each eNB has different radio environment andscheduling policy, PHR operation is independently performed byconfiguring PHR parameters such as periodicPHR-Timer, prohibitPHR-Timer,and dl-PathlossChange per MAC entity.

2) PHR transmission: in dual connectivity, it is generally assumed thatexchanging scheduling information between different eNBs over X2non-ideal backhaul in real time would not be possible. Without thelatest scheduling information of other eNB, the PH information ofserving cells under control of other eNB might not be so useful becausePH information cannot be understood or interpreted precisely withoutscheduling information. However, as the UE has limited power in uplink,PH information of serving cells of other eNB is still considered asuseful as long as it provides any rough information on power status inUE. Therefore, in dual connectivity, when reporting PH to one eNB, thePH information of all activated serving cells is included in the PHR MACCE.

3) PHR triggering: currently, PHR is triggered by five events asspecified in above mentioned. As PHR itself can be seen as overhead, PHRshould not be transmitted wastefully. Thus, it can be considered asnatural to trigger PHR only in the MAC entity where the serving cellwhere PHR trigger event is met is belonging to. If PHR is triggered inone MAC entity, it does not necessarily mean that PHR should betriggered in other MAC entity. On the other hand, if PHR is triggered inone MAC entity, it may imply that power status in the serving cells inother eNB.

FIG. 14 is a conceptual diagram for triggering power headroom reportingin a dual connectivity system according to embodiments of the presentinvention.

For a UE configured with at least two MAC entities to connect to atleast two eNBs, i.e., one MAC entity for one eNB, in order to avoidexcessive PHR transmission by the UE, it is invented that when a PHRtrigger event occurs in one MAC entity, the PHR is triggered in the MACentity or in all MAC entities depending on the type of PHR triggerevent. In detail, if PHR trigger event belongs to the first type, thePHR is triggered in the MAC entity where the PHR trigger event occurs.Otherwise, if PHR trigger event belongs to the second type, the PHR istriggered in all MAC entities configured to the UE.

In detail, the UE configures a plurality of MAC entities (S1401), eachMAC entity corresponding to each eNB.

When an event for triggering PHR in a MAC entity of the plurality of MACentities (S1403) occurs, the UE checks whether a type of the event fortriggering PHR in a MAC entity is a first type or a second type (S1405).

If the type of the event is the first type event, the PHR is triggeredin the plurality of MAC entities (S1407). And a plurality of PHR MAC CEsis generated, wherein the each PHR MAC CE is for each MAC entity(S1409). The respective plurality of PHR MAC CEs is transmitted torespective eNBs (S1411).

Preferably, the first type comprises i) a prohibitPHR-Timer of the MACentity expires or the prohibitPHR-Timer of the MAC entity has expiredand and the path loss has changed more than dl-PathlossChange dB for atleast one activated Serving Cell belonging to the MAC entity which isused as a pathloss reference since the last transmission of a PHR to thecorresponding eNB when the UE has UL resources for new transmission(Event 1), ii) an SCell belonging to the MAC entity with configureduplink is activated (Event 4), and iii) the prohibitPHR-Timer of the MACentity has expired when the UE has UL resources for new transmission toa corresponding eNB and the following is true in this TTI for any of theactivated Serving Cells belonging to the MAC entity with configureduplink (Event 5): there are UL resources allocated for transmission orthere is a PUCCH transmission on this cell, and the required powerbackoff due to power management (as allowed by P-MPR_(c)) for this cellhas changed more than dl-PathlossChange dB since the last transmissionof a PHR to the corresponding eNB when the UE had UL resources allocatedfor transmission or PUCCH transmission on this cell.

Preferably, the first type event is related to cell status change.

Preferably, the first type event is further comprises addition of aPSCell. Because the PSCell is also SCell, that the PSCell is added iscorresponding to that the SCell belonging to the MAC entity withconfigured uplink is activated. That is, Event 4 can include the casethat PSCell is added. The PSCell is added after the PSCell is activated.

If the type of the event is the second type event, the PHR is triggeredin the MAC entity (S1413). And a PHR MAC CE is generated for the MACentity (S1415). The generated PHR MAC CEs is transmitted tocorresponding eNB (S1417).

Preferably, the second type comprises i) a periodicPHR-Timer of the MACentity expires (Event 2) and ii) upon configuration or reconfigurationof the power headroom reporting functionality for the MAC entity byupper layers, which is not used to disable the function (Event 3).

FIGS. 15 and 16 are conceptual diagrams for triggering power headroomreporting corresponding to a first type according to embodiments of thepresent invention.

In case of the first type, the PHR is triggered in all MAC entitiesconfigured to the UE when an event for triggering the PHR occurs in aMAC entity (steps of S1403 to S1407).

Thus, in case that the UE configures a first MAC entity and a second MACentity (S1501), the UE triggers PHR in the second MAC entity as well asin the first MAC entity (S1505) if a prohibitPHR-timer for the first MACentity expires (S1503).

Thus, in case that the UE configures a first MAC entity and a second MACentity (S1601), the UE triggers PHR in the second MAC entity as well asin the first MAC entity (S1605) if at least one SCell for the first MACentity is changed from deactivate status to activate status (S1603).

In conclusion, a power headroom report (PHR) shall be triggered if anyof the following events occur:

i) prohibitPHR-Timer expires or has expired and the path loss haschanged more than dl-PathlossChange dB for at least one activatedserving cell of ‘any MAC entity’ which is used as a pathloss referencesince the last transmission of a PHR in this MAC entity when the MACentity has UL resources for new transmission;

ii) periodicPHR-Timer expires;

iii) upon configuration or reconfiguration of the power headroomreporting functionality by upper layers, which is not used to disablethe function;

iv) activation of an SCell of ‘any MAC entity’ with configured uplink;

v) addition of the PSCell; and

vi) prohibitPHR-Timer expires or has expired, when the MAC entity has ULresources for new transmission, and the following is true in this TTIfor any of the activated serving cells of ‘any MAC entity’ withconfigured uplink: there are UL resources allocated for transmission orthere is a PUCCH transmission on this cell, and the required powerbackoff due to power management (as allowed by P-MPR_(c)) for this cellhas changed more than dl-PathlossChange dB since the last transmissionof a PHR when the MAC entity had UL resources allocated for transmissionor PUCCH transmission on this cell.

Because the PHR is performed independently in unit of MAC entity,respective PHR is performed independently unless the condition of ‘anyMAC entity’ is attached. For example, when the periodicPHR-Timer forfirst MAC entity expires, the PHR is triggered in only the first MACentity. Otherwise, when prohibitPHR-Timer for the first MAC entityexpires, the PHR is triggered in all MAC entities.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

The embodiments of the present invention described hereinbelow arecombinations of elements and features of the present invention. Theelements or features may be considered selective unless otherwisementioned. Each element or feature may be practiced without beingcombined with other elements or features. Further, an embodiment of thepresent invention may be constructed by combining parts of the elementsand/or features. Operation orders described in embodiments of thepresent invention may be rearranged. Some constructions of any oneembodiment may be included in another embodiment and may be replacedwith corresponding constructions of another embodiment. It is obvious tothose skilled in the art that claims that are not explicitly cited ineach other in the appended claims may be presented in combination as anembodiment of the present invention or included as a new claim bysubsequent amendment after the application is filed.

In the embodiments of the present invention, a specific operationdescribed as performed by the BS may be performed by an upper node ofthe BS. Namely, it is apparent that, in a network comprised of aplurality of network nodes including a BS, various operations performedfor communication with an MS may be performed by the BS, or networknodes other than the BS. The term ‘eNB’ may be replaced with the term‘fixed station’, ‘Node B’, ‘Base Station (BS)’, ‘access point’, etc.

The above-described embodiments may be implemented by various means, forexample, by hardware, firmware, software, or a combination thereof.

In a hardware configuration, the method according to the embodiments ofthe present invention may be implemented by one or more ApplicationSpecific Integrated Circuits (ASICs), Digital Signal Processors (DSPs),Digital Signal Processing Devices (DSPDs), Programmable Logic Devices(PLDs), Field Programmable Gate Arrays (FPGAs), processors, controllers,microcontrollers, or microprocessors.

In a firmware or software configuration, the method according to theembodiments of the present invention may be implemented in the form ofmodules, procedures, functions, etc. performing the above-describedfunctions or operations. Software code may be stored in a memory unitand executed by a processor. The memory unit may be located at theinterior or exterior of the processor and may transmit and receive datato and from the processor via various known means.

Those skilled in the art will appreciate that the present invention maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent invention. The above embodiments are therefore to be construedin all aspects as illustrative and not restrictive. The scope of theinvention should be determined by the appended claims and their legalequivalents, not by the above description, and all changes coming withinthe meaning and equivalency range of the appended claims are intended tobe embraced therein.

What is claimed is:
 1. A method for a User Equipment (UE) operating in awireless communication system, the method comprising: configuring the UEwith a first Medium Access Control (MAC) entity corresponding to a firste-NodeB (eNB) and a second MAC entity corresponding to a second eNB fora radio bearer, wherein a power headroom reporting (PHR) of differentMAC entities in the UE operate independently; triggering PHRs andgenerating PHR MAC control elements (CEs) in the first MAC entity andthe second MAC entity respectively, if a first type event occurs in thesecond MAC entity; triggering a PHR and generating a PHR MAC CE in thesecond MAC entity only, if a second type event occurs in the second MACentity; and transmitting the PHR MAC CE generated by the first MACentity to the first eNB, or transmitting the PHR MAC CE generated by thesecond MAC entity to the second eNB, wherein the first type eventincludes: there is an uplink (UL) transmission on any cell of the secondMAC entity, and a required power backoff for the any cell of the secondMAC entity has changed more than a threshold, and wherein the secondtype event comprises at least one of the following events: aperiodicPHR-Timer of the second MAC entity expires, or upon a powerheadroom reporting functionality for the second MAC entity is configuredor reconfigured by upper layers.
 2. The method of claim 1, wherein thefirst type event further includes: the required power backoff for theany cell of the second MAC entity has changed more than the thresholdsince a last transmission of a PHR.
 3. A User Equipment (UE) operatingin a wireless communication system, the UE comprising: a Radio Frequency(RF) module; and a processor operably coupled with the RF module andconfigured to: configure the UE with a first Medium Access Control (MAC)entity corresponding to a first e-NodeB (eNB) and a second MAC entitycorresponding to a second eNB for a radio bearer, wherein a powerheadroom reporting (PHR) of different MAC entities in the UE operateindependently, trigger PHRs and generate PHR MAC control elements (CEs)in the first MAC entity and the second MAC entity respectively, if afirst type event occurs in the second MAC entity, trigger a PHR andgenerate a PHR MAC CE in the second MAC entity only, if a second typeevent occurs in the second MAC entity, and transmit the PHR MAC CEgenerated by the first MAC entity to the first eNB, or transmit the PHRMAC CE generated by the second MAC entity to the second eNB, wherein thefirst type event: there is an uplink (UL) transmission on any cell ofthe second MAC entity, and a required power backoff for the any cell ofthe second MAC entity has changed more than a threshold, and wherein thesecond type event comprises at least one of the following events: aperiodicPHR-Timer of the second MAC entity expires, or upon a powerheadroom reporting functionality for the second MAC entity is configuredor reconfigured by upper layers.
 4. The UE of claim 3, wherein the firsttype event further includes: the required power backoff for the any cellof the second MAC entity has changed more than the threshold since alast transmission of a PHR.
 5. The method of claim 1, wherein if thefirst type event occurs in the second MAC entity, the PHR is triggeredin the first MAC entity when a prohibitPHR-timer of the first MAC entityexpires, and the PHR is triggered in the second MAC entity, when aprohibitPHR-timer of the second MAC entity expires.
 6. The UE of claim3, wherein if the first type event occurs in the second MAC entity, thePHR is triggered in the first MAC entity when a prohibitPHR-timer of thefirst MAC entity expires, and the PHR is triggered in the second MACentity, when a prohibitPHR-timer of the second MAC entity expires.